1. Field of the Invention
The invention is related to the field of measurement of thermal neutron capture cross-section of earth formations penetrated by a wellbore. More specifically, the invention is related to methods for determining the thermal neutron capture cross-section of the earth formations where the measurements are corrected for neutron diffusion and the effects of salinity of fluid in the wellbore.
2. Description of the Related Art
A number of well logging instruments are known in the art for measuring the thermal neutron capture cross-section (or its inverse, the thermal neutron "die-away" or "decay" time) of earth formations. See U.S. Pat. No. 4,041,309 issued to Hopkinson, for example. These well logging instruments typically include a controllable source of high energy neutrons. Some types of thermal neutron capture cross-section instruments include a single gamma ray photon detector spaced apart from the source along the instrument. The neutron source emits controlled-duration "bursts" of high energy neutrons into the earth formations surrounding the instrument. The high energy neutrons interact with atomic nuclei in the formations, decreasing in energy with such interactions until they reach the "thermal" energy level (generally defined as an average energy of about 0.025 electron volts), whereupon they may be absorbed, or "captured", by certain atomic nuclei in the earth formations which have a relatively high tendency to capture thermal neutrons. When such a nucleus captures a thermal neutron, it emits a gamma ray (called a "capture gamma ray") in response. The capture gamma rays are detected by the gamma ray photon detector. The rate at which the numbers of detected capture gamma rays decreases with respect to the elapsed time after the end of the neutron burst is related to the capture cross-section of the particular earth formation, among other things.
The thermal neutron capture cross-section as determined from the counts of gamma rays made by a single detector generally has to be corrected for the effect of the salinity of the fluid in the wellbore and for the effect of neutron diffusion. Various corrections have been devised for the single-detector type instrument based on laboratory measurements of the response of such instruments to various known salinity fluids in a simulated wellbore. Using the laboratory-derived correction methods on measurements made in an actual wellbore requires knowledge of the salinity of the wellbore fluid, which can be difficult and expensive to determine. It should be noted that it is common practice to measure the electrical resistivity of the fluid in the wellbore, but as is well known in the art, the electrical resistivity is only partially related to the concentration of sodium chloride (the salinity) in the wellbore fluid, since other chemical components may be present in the fluid which affect the overall electrical conductivity of the fluid but not its capture cross-section. The salinity of the wellbore fluid can have a pronounced effect on the measurements of capture cross-section of the earth formation because chlorine nuclei have a very high capture cross-section.
Improvements to the measurements obtained using a single-detector instrument include providing a second gamma ray detector on the instrument spaced further away from the source than is the first detector. Measurements of capture gamma rays from the second detector can be used to provide some correction to the measurements made by the first detector in determining the thermal neutron capture cross-section of the earth formations. U.S. Pat. No. 4,445,033 issued to Preeg et al describes such an instrument and a method for processing the measurements from both detectors to obtain "corrected" capture cross-section measurements. The method and apparatus described in the Preeg et al '033 patent, however, still requires knowledge of the salinity of the fluid in the wellbore.
Another two-detector technique for determining neutron capture cross-section is described in U.S. Pat. No. 3,509,342 issued to Dewan. The method described in this patent is intended to correct the capture cross-section measurements for the fractional volume of pore space ("porosity") in the earth formations. The method in the Dewan '342 patent, however, does not account for the effects of the fluid in the wellbore, which effect is strongly related the salinity of the fluid in the wellbore.
Still another method for determining neutron capture cross-section is described in U.S. Pat. No. 5,235,185 issued to Albats et al. The method described in the Albats et al '185 patent uses measurements from a gamma ray detector and a detector sensitive primarily to thermal neutrons in order to provide diffusion correction to the capture cross-section measurements. The correction method described in the Albats et al '185 patent, however, requires knowledge of the salinity of the fluid in the wellbore, the size (local diameter) of the wellbore in the vicinity of the logging instrument and the porosity of the earth formation.
The prior art does not disclose a method for determining the thermal neutron capture cross-section of earth formations which accounts for neutron diffusion and does not require explicit knowledge of the salinity of the fluid in the wellbore or the porosity of the earth formations.